Advancements in propulsion technologies have required gas turbine engines to operate at higher temperatures. This increase in operating temperature has required concomitant advancements in the operating temperatures of metallic (e.g. nickel and cobalt base superalloy) turbine engine components to withstand oxidation and hot corrosion in service. Inwardly grown platinum modified (platinum-bearing) diffusion aluminide coatings have been formed on superalloy turbine engine components to meet these higher temperature requirements. One such inwardly grown platinum modified diffusion coating is formed by chemical vapor deposition using aluminide halide coating gas and comprises an inward diffusion zone and an outer two phase [PtAl2+(Ni,Pt)Al] layer. The two phase Pt modified diffusion aluminide coatings are relatively hard and brittle and have been observed to be sensitive to thermal mechanical fatigue (TMF) cracking in gas turbine engine service.
Outwardly grown, single phase platinum modified (platinum-bearing) diffusion aluminide coatings also have been formed on superalloy turbine engine components to meet these higher temperature requirements. One such outwardly grown platinum modified diffusion coating is formed by chemical vapor deposition using a low activity aluminide halide coating gas as described in U.S. Pat. Nos. 5,658,614; 5,716,720; 5,989,733; and 5,788,823 and comprises an inward diffusion zone and an outer (additive) single phase (Ni,Pt)Al layer.
U.S. Pat. No. 6,589,668 describes a method of forming a Pt-free outwardly-grown, single phase diffusion aluminide coating on an airfoil of a gas turbine engine blade and a graded platinum, outwardly grown, single phase diffusion aluminide coating on damper pocket surfaces using multiple aluminum sources in a coating chamber.